Some general stuff about xmitting chokes: First, like with cholesterol,
there are two kinds of self resonances--good and bad. The good resonance is
the parallel and the bad is the series. In fact, for the highest perfomance
possible, a single-band amp's plate choke should be naturally parallel
resonant on that band. Parallel resonance is measured without a short
applied across the choke. Series resonance is measured with the short. Both
resonance modes are ideally measured with the choke "in place" in the PA
compartment.
Parallel resonance is analogous to using a tuned choke as the choke input
filter in a power supply. The effective Z is much higher than the inductive
reactance would be of the un-tuned choke alone.
Impedances of solenoid chokes are very high because of natural parallel
resonances. In fact, there are multiple parallel resonances because of the
structure of the solenoid. The main, fundamental resonance in a practical
choke can exhibit an EFFECTIVE parallel reactance of up to 1-megohm. Useful
areas around the main peak commonly have effective reactances of 200,000
ohms. This is how solenoid chokes "work."
These effective reactances are far, far in excess of simple inductance
values given by manufacturers or by calculations for inductance, which are
LOW FREQUENCY values only.
To prove this without a doubt, calculate the inductive reactance only, using
the simple choke inductance value as specified in typical amps. About 1 mH?
Now, for 1.8 mHz, that's around 12,000 ohms? Far from 200,000 or a million.
And with say 4 kV swinging at the "top" that equals excessive circulating
currents within the choke. Fortunately, that's not the case.
It's the self-resonance (good kind) that is in service for us.
Hit me. 73, K6XK
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